The variations of nucleotides and related compounds in chattonella antiqua (Hada) Ono, Alexandrium catenella (Whedon et Kofoid) Balech and Scrippsiella trochoidea (Stein) Loeblich III were determined during growth in laboratory culture using ESM-enriched seawater. The results clearly showed that total cellular contents of nucleotides and related compounds of these phytoflagellates increased greatly during lag and early log phases, after which they decreased markedly and became approximately constant during the stationary phase. The nucleotides and related compounds in cysts of both A.catenella and S.trochoidea were higher than those of vegetative cells during the stationary phase. The intracellular GTP/ATP ratios in C.cntiqua during the log phase were lower than that of other phases of growth. The GTP/ATP ratios of S.trochoidea were much greater than that of A.catenella through the all stages of the culture. In the cyst of S.trochoidea, the GTP/ATP ratio was higher than that in vegetative cell. The value of energy charge [EC=(ATP+1/2ADP)/(ATP+ADP+AMP)] of C.antiqua decreased notably after inoculation into a new culture medium. It was maintained at the value of about 0.6 during the log phase, and after cessation of growth it declined slowly to a value of about 0.5. In A.catenella and S.trochoidea, the EC values increased remarkably after the inoculation. The EC values in cysts of A.catenella and S.trochoidea were lower than those in vegetative cells. These results may depend on the lower metabolic activity in cyst than that in vegetative cell.